1. Field of the Invention
The invention relates to a planetary gearset, the main components of which are a sun gear, a ring gear, and a carrier that retains pinion gears (planet gears) arranged between the sun gear and the ring gear.
2. Description of the Related Art
Typically, a planetary gearset is a device in which a sun gear, which is a gear with external teeth, and a ring gear, which is a gear with internal teeth, are arranged on the same axis, with pinion gears arranged between the sun gear and the ring gear, which are retained by the carrier so as to be able to rotate and revolve. With respect to these pinion gears, a device provided with pinion gears that engage with both the sun gear and the ring gear at the same time has been known. In addition, a device provided with a first pinion gear that engages with the sun gear, and a second pinion gear that engages with the first pinion gear and the ring gear has also been known.
The former planetary gearset is commonly referred to as a single pinion type planetary gearset, while the latter planetary gearset is commonly referred to as a double pinion type planetary gearset. Moreover, a Ravigneaux type planetary gearset, in which a single pinion type planetary gearset is integrally combined with a double pinion type planetary gearset, has also generally been known.
A planetary gearset of this type of configuration has three main elements, which are a sun gear, a ring gear, and a carrier. The planetary gearset functions as a decelerating device, an accelerating device, and a reversing device and the like by making one of those elements an input element, another one an output element, and another one a fixed element. Coupling together any two of the elements integrates the entire planetary gearset.
Regardless of the mode of use, the pinion gears become the medium through which torque is transmitted between the sun gear and the ring gear. Accordingly, the more pinion gears there are, the more torque that can be transmitted between the sun gear and the ring gear. Therefore, the invention disclosed in Japanese Patent Laid-Open Publication No. 4-175542, for example, increases the number of pinion gears that can be retained by the carrier by changing the structure of the carrier.
In this publication it is disclosed that, assuming that the torque applied to each of the plurality of pinion gears is equal, when five instead of four pinion gears are provided, the torque able to be transmitted increases by 1.25 times. Depending on the mode of use of the planetary gearset, however, a large load may end up being applied to a specific pinion gear, thereby leading to a decrease in strength and durability of the planetary gearset.
More specifically, FIGS. 33 and 34 show an example of two pinion gears 3 arranged between a sun gear 1 and a ring gear 2, with a carrier 4 that retains those pinion gears 3 fixed by splines 6 to a casing 5. In addition, a counter gear 8 is engaged with external teeth 7 formed on an outer peripheral face of the ring gear 2. In addition, the ring gear 2 is supported by the casing 5 via a bearing 9 fitted at the outer periphery of the ring gear 2. In this kind of mode of use, a load F from the transmission of torque acts between the ring gear 2 and the counter gear 8. In FIG. 33, the pressure angle is ignored, so the load F acts in a direction at a right angle to a virtual line connecting the center of the sun gear 1 with the center of the counter gear 8.
Meanwhile, with respect to the fact that there is an inevitable gap e between the casing 5 and the bearing 9 that supports the ring gear 2, a carrier 4 is fixed to the casing 5, so a reaction force against the load F acts between the pinion gears 3, which are supported by the carrier 4, and the ring gear 2. Further, a load following the transmission of torque acts on each of the pinion gears 3.
Accordingly, the relationship between forces f1 and f2 that act on the pinion gears 3 and the load F is as shown by the arrows in FIG. 33, with the amount being:f1=(F/2)*{(R/r)+1}f2=(F/2)*{(R/r)−1},wherein the radii r and R of the contact point between the pinion gears 3 and the ring gear 2 is the radius of the location upon which the load F acts.
The pinion gears 3 also mesh with the sun gear 1 and transmit the torque thereto, so the bearings of the pinion gears 3 receive a radial load that is two times that of each of the forces f1 and f2.
In this way, the force f1 acting on the pinion gear 3 that is closest to the contact point between the ring gear 2 and the counter gear 8 is larger than the force f2 acting on the other pinion gear 3. As a result, the strength or durability of the pinion gear 3 upon which the load is largest restricts the strength or durability of the overall planetary gearset.
This is true even when another element is fixed instead of the carrier 4 and torque is transmitted between another element that is not fixed and the rotating member outside of the planetary gearset. That is, the related art does not take into consideration the effect of the load generated by the transmission of torque between the rotating member outside of the planetary gearset and the planetary gearset. As a result, with the related art, the strength and durability of the overall planetary gearset is restricted by the strength and durability of a specific member.
Further, because the sun gear and the ring gear in the type of planetary gearset described above are rotatably arranged on the same axis, the bearings retaining these gears can be fixed on the outer periphery of a predetermined shaft or fixedly fitted to a boss portion that is integrated with the casing. As a result, those bearings can be lubricated relatively easily by supplying lubricating oil via the shaft or the boss portion.
On the other hand, because the pinion gears have a comparatively smaller diameter than the sun gear and the ring gear and transmit torque between those two gears, the pinion gears rotate quickly and receive a large load. In addition, these pinion gears are fitted via bearings to pinion pins attached to the carrier, so those bearings are separated from the bearing that supports the sun gear and the bearing that supports the ring gear.
In this way, there are times when the load and speed conditions on the bearings that support the pinion gears are severe, so it is important that they be sufficiently lubricated. In this case, the bearings of the pinion gears can be lubricated by supplying lubricating oil to the rotational center of the carrier when the carrier is rotated. This lubricating oil then reaches the bearings of the pinion gears by centrifugal force from the rotation of the carrier, and lubricates them. However, in the planetary gearset, one of the rotating elements is often used as a fixed element. When the carrier is made that fixed element, the centrifugal force used in the lubrication of the pinion gears retained by that carrier is no longer available.
Here, Japanese Patent Laid-Open Publication No. 2001-227625 discloses a device in which a lubricating oil path forming member is fixed to one side of a carrier that is fixed to a case, the device supplying lubricating oil to the carrier via an oil path formed in the case and the lubricating oil path forming member. According to this construction, lubricating oil can be supplied to the carrier (or more correctly, to the pinion bearings) arranged between the rotational axis of the planetary gearset and an inner peripheral face of the case.
With the device disclosed in the aforementioned publication, lubricating oil is supplied to each of the pinion bearings by running naturally down the lubricating oil path. With respect to the lubricating oil formed in the vertical direction, however, oil holes leading the lubricating oil to the pinion bearings are formed orthogonal to the lubricating oil path, which makes it difficult to lead the lubricating oil running downward into the oil holes. As a result, the lubricating oil that has run down ends up collecting on the lower side. The lubricating oil collects until it reaches the height of the open edge of the oil holes, after which it enters the oil holes and is supplied to the pinion bearings.
Therefore, because the carrier does not revolve, the pinion bearing that is stopped on the upper side is not lubricated until the lubricating oil rises, which takes a long time. Further, when only a small amount of the lubricating oil runs down, a sufficient amount of lubricating oil does not collect so the level thereof does not rise, which may result in the problem of lubricating oil not being able to be supplied to the pinion bearing on the upper side.
Furthermore, a lubrication system is generally known that supplies lubricating oil to portions where there is friction and portions where heat is generated in a planetary gearset or the like such as that described above. Also, even in a typical gear mechanism that is not a planetary gearset, a lubrication system is widely known that lubricates and cools by kicking up lubricating oil to portions where there is friction and portions where heat is generated. One example of such a device is disclosed in Japanese Patent Laid-Open Publication No. 7-217725. In a differential gear disclosed in this publication, a drive shaft and a ring gear are rotatably provided in a differential carrier. Further, a hypoid gear formed on the drive shaft is engaged with the ring gear, and the drive shaft is supported by a bearing. Moreover, a lubricating oil sump is provided in the differential carrier, and a portion of the ring gear is submersed in the lubricating oil sump. Also, an oil reservoir with an inflow opening is provided inside the differential carrier above the drive shaft. An outflow opening is also formed above the drive shaft in the oil reservoir.
In the lubrication system of the aforementioned publication, when torque from the drive shaft is transmitted to the ring gear, the ring gear rotates and kicks up lubricating oil from the lubricating oil sump. When this happens, some of the lubricating oil adhered to the ring gear is thrown by centrifugal force in a direction tangential to the ring gear. This lubricating oil passes through the inflow opening and runs into the oil reservoir, after which it naturally drips down from the outflow opening to lubricate and cool the bearings.
In the lubrication system described in this publication, however, the lubricating oil is supplied to the oil reservoir in only one transfer step, being the transfer of the lubricating oil in the lubricating oil sump by the rotation of the ring gear. Therefore, when the oil reservoir and the ring gear are separated by a large distance, there may be a decrease in the amount of lubricating oil supplied to the oil reservoir. More specifically, when the rotation speed of the ring gear drops below a predetermined rotation speed, the centrifugal force, which is used to throw the lubricating oil, is weak, making the aforementioned problem even more pronounced. As a result, the degree of freedom of the layout of the oil reservoir and the ring gear is reduced. In order to solve the foregoing problems, the amount of lubricating oil kicked up by the rotation of the ring gear can be increased by increasing the surface area of the ring gear that is submersed in the lubricating oil sump. However, this creates another problem of increased power loss during the rotation of the ring gear due to the shearing resistance of the lubricating oil.